Start of summer thaw means fish (5)
I believe the answer is:
smelt
'fish' is the definition.
(smelt is a kind of fish)
'start of summer thaw' is the wordplay.
'start of' says to take the initial letters.
'thaw' becomes 'melt' ('melt' can be a synonym of 'thaw').
The first letter of 'summer' is 's'.
's'+'melt'='SMELT'
'means' is the link.
(Other definitions for smelt that I've seen before include "Got a whiff of" , "whiffed" , "emitted noxious effluvium" , "Stank" , "Food fish often fried" .)